F.4A.mathsx1

Note that the │y-coordinate│of the intersection pt. of L1 and L2 is the height of the triangle
L1:X+Y=3---(1)
L2:X-Y=K---(2)
(1)-(2) gives Y=(3-K)/2
the x-intercept of L1=3
the x-intercept of L2=K
Since the area of the triangle bounded by L1,L2,L3 is 4 sq.units
(│3-K││(3-K)/2│)/2=4
(3-K)^2/2=8
(3-K)^2=16
3-K=4 or 3-K=-4
K=-1 or K=7